1. Field of the Invention
The present invention relates to a metal gasket used to prevent leak of fluid in an ultra-high vacuum device used in a semiconductor manufacturing apparatus, a nuclear plant, and the like.
2. Description of the Related Art
Various types of metal gaskets have been used to obtain high airtightness in a semiconductor manufacturing apparatus and the like.
Among them, a metal hollow O-ring gasket is generally made by forming a metal pipe of stainless steel, inconel, and the like in a ring shape by bending processing and the like and welding both the ends of the metal pipe to each other. In the metal hollow O-ring gasket having the above structure, excellent sealing can be obtained by deforming the metal ring by applying a strong tightening pressure to it.
However, since the metal hollow O-ring gasket is made by welding the two ends of the metal pipe formed in the ring-shape by bending processing as described above, flashes produced at welding remain usually inside and outside of the pipe. Since the flashes outside of the pipe are treated to be eliminated by cutting, grinding, and the like, the thickness of the pipe more or less decreases. Therefore, compressive strength at the welded portion can be different from that at the other part when the gasket is tightened. In the case where it is mounted for use where an ultra-high vacuum is required, leakage may be produced at the welded portion where the thickness is decreased.
Further, when the gasket has a very small diameter of, for example, 10 mm, a problem arises in that it is difficult to bend and weld it as well as there is also a disadvantage that a very strong bolt is required to apply a strong tightening force.
To cope with the above problems, an approximately C-shaped metal C-ring gasket G6 is used to reduce a welded portion by making the gasket by bending a metal sheet or a metal tube and to form the gasket easily even if its diameter is small as shown in FIG. 11.
Further, there are also used a metal C-ring gasket having a coil spring or the like disposed therein to increase an amount of restoration and a metal C-ring gasket G7 having an approximately K-shaped cross section as shown in FIG. 12 that is made by cutting a metal tube.
Although the metal ring gaskets described above have been used particularly in a gas supply line of a semiconductor manufacturing apparatus, at present, SEMI (Semiconductor Equipment and Materials International) intends to standardize the gas supply line as an integrated gas system to achieve miniaturization.
As the characteristics of a gasket used in the integrated gas system, it is required that even if the gasket is replaced 20 times or more on the same flange, it can be used for sealing in addition to that the gasket can maintain ultra-high vacuum of 1×10−11 Pa·m3/secHe or less. In the integrated gas system, the gasket is interposed between various components such as a flange, a valve, a filter, and the like that constitute a gas passage and secured by bolts. However, it is required to reduce the tightening force of the gasket necessary to seal because the diameter of the bolt small and it is difficult to apply a large force to the bolt.
In contrast, Japanese Patent Application Publication No. 6-56211 reduces a tightening force necessary to seal by using a metal C-ring gasket G8 with a spring and projections, in which a metal spring 3 is contained in metal covers 1 and 2 and circumferential projections 4 are formed on supporting surfaces acting as the sealing surfaces of the metal covers as shown in FIG. 13, and Japanese Patent Application No. 2001-401880 filed by the applicant reduces a tightening force necessary to sealing by using a metal gasket G5 having annular projections a1 formed on the surfaces thereof as shown in FIG. 6.
However, since these metal C-ring gaskets with the projections secure a sealing property by generating concentrated loads through the small projections when the gaskets are used, they are disadvantageous in the following points. That is, when the size of the projections is larger than an optimum size, the sealing property may be lowered because the sealing surfaces of flanges are damaged by the projections, whereas when the size of the projections is smaller than the optimum size, a surface pressure necessary to sealing cannot be maintained because the gasket comes into contact with the sealing surfaces of flanges through the entire surfaces thereof. In addition to the above, it is very difficult in manufacturing to manage the shape of the small projections.